You need a bit more than checksums for this. If anything the FSF published many different versions of the "official" GPL2 texts and this would defeat checksums. See https://github.com/pombredanne/gpl-history ... so in the general cases hashing a text does not work consistently and safely. Eventually you need a diff for a pairwise comparison, and the only difficulty is making diff fast or find ways to approximate it fast enough to avoid doing a full diff.